1. Field of the Invention
The present invention is related to a USB circuit provided in a semiconductor integrated circuit.
2. Description of Related Art
Conventionally, USB (Universal Serial Bus) is known as a standard for connecting a host such as a personal computer and peripheral equipment such as printers, scanners, modems and the like. USB is a standard that is designed for unifying the connection interfaces of the various peripheral equipment. Consequently, employment of the USB standard makes possible to use a single type interface regardless of the type of peripheral equipment.
A USB circuit includes a USB controller and a USB driver. The USB controller and USB driver are separately mounted on the same semiconductor chip. The USB controller and USB driver are connected using an input signal line pair and an output signal line pair. The USB driver is also connected by different pairs of signal lines to the USB connector being provided with the case of the equipment.
A USB signal comprises a pair of signals D+ and D−. In the USB standard, the value of the signals D+ and D− must always change at the same phase. As a result, the propagation delay for both of the input signal line pair, as well as for both of the pair of output signals lines, must be equal. If a phase difference occurs with respect to a change in the signals D+ and D−, errors may occur in the operation of the host or peripheral device using the USB interface.
For matching the propagation delay for each pair of signal lines, the layout of the semiconductor integrated circuit must be designed to ensure that the lengths of the two signal lines are identical. Moreover, the number of gates in the USB driver must be made identical. However, it is not easy to make the lengths of the signal lines perfectly equal, which significantly increases the difficulty of designing. Furthermore, even where the design succeeds in making the lengths of the pair of signal lines perfectly equal, during actual operation of the semiconductor integrated circuit phase difference sometimes occurs between the signals D+ and D− due to a change in the external operating conditions, such as a fluctuation in the power supply voltage or the ambient temperature.
In recent years, it has become popular to manage semiconductor integrated circuits as IP (Intellectual Property) blocks. An IP block is a circuit block that is bought and sold commercially as intellectual property. For example, when an high degree of system LSI (Large-Scale Integrated Circuit) or the like is to be made into a commercial product, it is not easy for a single semiconductor manufacturer to develop all of the circuit blocks on the LSI. On the other hand, where IP blocks are marketed between corporations, the cost of development can be reduced and the lead time for development can be shortened.
In order to increase the commercial value of an IP block, it is required to minimize the design modification of the IP block according to kinds of integrated circuit chip on which the IP block is mounted, as well as minimize influences on the designs of other circuit blocks. However, because the conventional USB circuit must be designed such that the lengths of the signal lines match perfectly, it occurs frequently that design of the USB circuit block becomes to need to be modified substantially and designs of other blocks become to need to be modified under influence of the USB circuit.